1. Field of the Invention
The present invention relates to a pre-pit detecting apparatus for detecting a pre-pit by which various control information such as address information or the like is recorded in advance, on an information record medium which is repeatedly recordable such as a DVD rewritable (hereinbelow, it is referred to as a DVD-RW) among information record mediums of high density recording types represented by a DVD.
2. Description of the Related Art
On the information record medium such as a DVD-RW, at a pre-formatting stage in advance of the actual recording operation, the pre-pits for carrying various control information such as address information etc., are formed on the disc. Then, when performing the recording or reproducing operation with respect to the information record medium, the control information required for the operation can be obtained by detecting the pre-pits.
For example, in case of the DVD-RW, information pits are formed on a groove track by modulating the reflection coefficient in dependence upon the crystal status or the amorphous status of a recording layer while the pre-pits are formed in advance as phase pits by concave and convex shapes on a land track.
By the way, in an optical pickup for use in reproducing and recording the information with respect to the information record medium, a light receiving element of four-divided type is used so as to realize the focusing servo by means of the astigmatism method and the tracking servo by means of the push-pull method. Therefore, the detection of the pre-pit is performed by using a light receiving element of such type.
For example, a reflection light of a beam spot emitted on the groove track is divided into four light detection areas at the light receiving surface of the light receiving element, and four light detection signals corresponding to these four light detection areas are obtained. Then, a pre-pit formed on the land track is detected by calculating a predetermined function of these detection signals.
However, in case of the information record medium such as a DVD-R for example, the information pit and the pre-pit are constructed such that the substrate is shaped in concave and convex shapes i.e., both of the pre-pit and the information-pit are formed as phase pits so as to give phase differences to the light beam and that the light intensity modulated by the diffraction at the phase pit is detected. Therefore, the detection timing is varied for each of the four light detection areas of the light receiving element, in which the positions of these four light detection areas are positioned with a shift or displacement in a forward or backward direction to each other along the track and/or in an inner or outer direction to each other along the direction across the track (i.e., along the radial direction of the disc), due to the difference in the characteristics such as a diffraction efficiency, a diffraction angle and so on at the phase pit.
Thus, the detection timing of the information pit at the four light detection areas may be delayed or precedent to each other. In addition, the detection signals of the four light detection areas may be changed in directions different from each other i.e., may be in such a direction to reduce the light intensity or to increase the light intensity. Accordingly, the peaks of the changing portions in the detection signals due to the pre-pit are not coincident in a time axis with each other because of the delay time. As a result, the pre-pit detectable range i.e., the detection margin, in which a threshold to detect the pre-pit is appropriately set, is in fact reduced. By this reduction of the detection margin, the pre-pit detection accuracy is certainly degraded, and the possibility of an erroneous operation due to a noise is increased by the degradation, which is a problem.